Present day network systems, such as local area (LAN), metropolitan area (MAN), and wide area (WAN), generally utilize electrical signal transmission arrangements wherein switching, modulation, demodulation, and the like are all performed electrically. However, the emphasis is now upon developing optical arrangements for such networks because of the numerous advantages inherent in optical signaling and transmission. Optical systems are generally immune to the effects of electrical radiation, do not produce electromagnetic interference (EMI), have no ground loop problems, and, generally, optical fiber has less signal attenuation over a given distance than co-axial cable.
A local network known as ALOHA is an example of a local area network (LAN) upon which much effort has been expended in converting it and derivatives thereof from electrical to optical transmission. The ALOHA network, based upon wireless transmission with a common radio frequency, was an early LAN approach in which stations were transmitting whenever they had information to communicate. "Collisions" occurred whenever there were two or more stations transmitting at the same time, not by design. A station did not "know" that it had transmitted successfully unless it received a confirmation or acknowledgment within a reasonably short time after the transmission.
Subsequent to the development of ALOHA was the development of a Media Access Control (MAC) protocol for use with stations connected to a common coaxial cable electrical bus using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) scheme which represented an improvement over the ALOHA scheme. With CSMA/CD, local area networks, for example, experience fewer collisions, and, when collisions do occur, a station detecting a collision directly will stop transmitting the remainder of its message packet. Statistical analysis shows that the CSMA/DC media access control protocol has twice the average through-put at optimum loading than ALOHA for the same speed of transmission. Also, with CSMA/CD, a station does not need to wait for a confirmation message to know that it has been successful in sending its message. This is a direct result of the collision detection feature. The station only transmits when the line is clear, and, having detected no collision during transmission, it can assume that the transmission was successful. A widely used form of LAN, Ethernet.RTM., operates on the CSMA/CD protocol. A station wishing to transmit listens to the transmission medium to determine if it is in use, i.e., if another transmission is in progress. If the medium, e.g., coaxial cable bus, is idle, the station transmits. However, if the medium is in use, the station waits for a predetermined randomly chosen period of time before trying again. A variation of the CSMA/CD mode is known as the 1-persistent protocol, wherein the station commences to listen, and, even if the medium is in use, continues to listen until the medium comes clear, at which time the station transmits. The 1-persistent protocol is analogous to a group of people seated around a table and conversing. If two stations, or persons, attempt to transmit or speak simultaneously a collision occurs, which they both detect, and they both stop transmitting or speaking, to try again after an elapsed time which is randomly different for the two stations, just as two people who attempted to speak at the same time would both stop, and delay trying to talk again for randomly delayed times.
Thus, if the "physical layer" of the communication network (the details of the transmission medium, signaling, receiving, and the ability to receive and detect collisions) can support the CSMA/CD MAC protocol, a fiber optic system promises better performance of a shared medium network than does an electrical system.
In the conversion of the Ethernet.RTM. or like systems to optical transmission, the individual stations have generally comprised a receiver and a separate transmitter which includes its own light source, i.e., laser or light emitting diode (LED), for example. With the stations so configured, and with each having a randomizable timing circuit, the advantages of Ethernet.RTM. or other CSMA/CD type local network can be realized to the fullest. The receiver "listens", while the transmitter waits in state of readiness. When the medium (optical bus) is clear, the light from the station light source is modulated with a signal and transmitted. Other stations along the bus receive the transmitted signals and wait their turn to transmit. Thus, an optical CSMA/CD local network is realized which utilizes the advantages of simplicity characteristic of an Ethernet.RTM. network, but which requires individual light sources and separate receivers at each station.